[Bug c++/100975] New: [C++23] Allow pointer to array of auto

[Bug c++/100975] New: [C++23] Allow pointer to array of auto

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

            Bug ID: 100975
           Summary: [C++23] Allow pointer to array of auto
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jason at gcc dot gnu.org
            Blocks: 98940
  Target Milestone: ---

https://wg21.link/cwg2397


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98940
[Bug 98940] Implement C++23 language features

[Bug c++/100975] [C++23] Allow pointer to array of auto

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-06-08

[Bug c++/100975] [C++23] Allow pointer to array of auto

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

--- Comment #1 from Jason Merrill <jason at gcc dot gnu.org> ---
  int a[3];
  auto (*p)[3] = &a;

[Bug c++/100975] [C++23] Allow pointer to array of auto

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Looks like just disabling the check in create_array_type_for_decl lets us
compile:

struct false_type { static constexpr bool value = false; };
struct true_type { static constexpr bool value = true; };
template<class T, class U>
struct is_same : false_type {}; 
template<class T>
struct is_same<T, T> : true_type {};

void
g ()
{
  int a[3];
  auto (*p)[3] = &a;
  auto (&r)[3] = a;
  static_assert (is_same<decltype (p), int(*)[3]>::value);
  static_assert (is_same<decltype (r), int(&)[3]>::value);
}

[Bug c++/100975] [C++23] Allow pointer to array of auto

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/100975] [C++23] Allow pointer to array of auto

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:e66d0b7b87d105d24da8c4784a0b907fb6b2c095

commit r12-1933-ge66d0b7b87d105d24da8c4784a0b907fb6b2c095
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jun 29 14:30:51 2021 -0400

    c++: DR2397 - auto specifier for * and & to arrays [PR100975]

    This patch implements DR2397, which removes the restriction in
    [dcl.array]p4 that the array element type may not be a placeholder
    type.  We don't need to worry about decltype(auto) here, so this
    allows code like

      int a[3];
      auto (*p)[3] = &a;
      auto (&r)[3] = a;

    However, note that

      auto (&&r)[2] = { 1, 2 };
      auto arr[2] = { 1, 2 };

    still doesn't work (although one day it might) and neither does

      int arr[5];
      auto x[5] = arr;

    given that auto deduction is performed in terms of function template
    argument deduction, so the array decays to *.

            PR c++/100975
            DR 2397

    gcc/cp/ChangeLog:

            * decl.c (create_array_type_for_decl): Allow array of auto.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/auto24.C: Remove dg-error.
            * g++.dg/cpp0x/auto3.C: Adjust dg-error.
            * g++.dg/cpp0x/auto42.C: Likewise.
            * g++.dg/cpp0x/initlist75.C: Likewise.
            * g++.dg/cpp0x/initlist80.C: Likewise.
            * g++.dg/diagnostic/auto1.C: Remove dg-error.
            * g++.dg/cpp23/auto-array.C: New test.

[Bug c++/100975] [C++23] Allow pointer to array of auto

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100975

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Implemented in GCC 12.